1. Field of the Invention
The present invention relates to a hydrogen absorbing material, which is capable of reversibly storing or releasing a hydrogen gas, a method for producing the same, and a hydrogen storage container which accommodates the hydrogen absorbing material therein.
2. Description of the Related Art
As is well known, a fuel cell generates electric power by supplying a fuel gas such as hydrogen to an anode, while supplying an oxygen-containing gas such as oxygen to a cathode. Therefore, for example, a fuel cell vehicle, which carries the fuel cell thereon, includes a gas storage container, which is filled with hydrogen. The fuel cell vehicle runs by using reaction gases made up of atmospheric air, which serves as the oxygen-containing gas, together with hydrogen, which is supplied from the gas storage container.
As clearly appreciated from this fact, the larger the hydrogen-accommodating amount of the gas storage container is, the longer the distance that the fuel cell vehicle can successfully run over is. However, if an excessively large gas storage container is carried by the vehicle, the weight of the fuel cell vehicle is increased. Consequently, an inconvenience arises in that the load on the fuel cell also is increased.
From this viewpoint, various trials have been made in order to improve the hydrogen-accommodating amount, while maintaining a small volume for the gas storage container. For example, it has been suggested in Japanese Laid-Open Patent Publication No. 2004-293571 to employ a tank (container) in which the pressure resistance thereof is improved, and which is filled with hydrogen gas at a relatively high pressure of 35 to 75 MPa.
Another technique for improving the hydrogen accommodating amount may be conceived of, in which a hydrogen absorbing or storing material, such as a hydrogen absorbing or storing alloy, is used, due to the following reasons. That is, as is well known, the hydrogen absorbing material is capable of occluding hydrogen depending on a temperature change, and also when the hydrogen absorbing material is accommodated within a container, the hydrogen accommodating amount is increased by an amount at which the hydrogen absorbing material is capable of occluding or storing hydrogen therein. For example, as described in Japanese Laid-Open Patent Publication No. 2004-18980, a hydrogen absorbing material containing AlH3 exhibits a large hydrogen occludable amount, and hence the hydrogen absorbing material is effective to improve the amount at which hydrogen is accommodated.
AlH3 releases hydrogen in accordance with the following formula (1), while AlH3 absorbs hydrogen in accordance with the following formula (2). Formulas (1) and (2) represent reactions that are caused at arbitrary absorbing/releasing sites, and do not imply that all of the AlH3 is oxidized/reduced.AlH3→Al+ 3/2H2  (1)Al+ 3/2H2→AlH3  (2)
AlH3, which is described in Japanese Laid-Open Patent Publication No. 2004-18980, is a crystalline material, in which sharp peaks appear, as may be clearly understood from the X-ray diffraction pattern shown in FIG. 2 of Japanese Laid-Open Patent Publication No. 2004-18980. In the case of crystalline AlH3, the hydrogen releasing reaction of formula (1) advances relatively easily, however, the hydrogen absorbing reaction of formula (2) does not advance easily at ordinary temperatures because the activation energy is large. If the hydrogen absorbing reaction of formula (2) is advanced at ordinary temperatures, by allowing H2 gas to make contact with absorbing/releasing sites that have been converted into Al, a high pressure of not less than several hundreds of MPa is required.
However, as described in paragraph [0004] of Japanese Laid-Open Patent Publication No. 2004-293571, the filling pressure of an ordinary tank is about 20 MPa. Moreover, the filling pressure is 75 MPa at a maximum in the case of the tank disclosed in Japanese Laid-Open Patent Publication No. 2004-293571. In the case of such a filling pressure, it is difficult to advance the hydrogen absorbing reaction of formula (2) at or in the vicinity of room temperature, which represents a temperature generally used by the fuel cell.